Magnetic disk drives are commonly employed in a variety of applications to store user data in a form that is substantially permanent. They employ magnetic disks or platters to magnetically record such data. Servo patterns are pre-written on the magnetic platter to define track positions and allow selection of a particular section of a track by a read/write head, which is part of a voicecoil actuator assembly. The read/write head is employed initially to write and then read the data, on request. As magnetic disk drive data densities increase, the writing of reliable servo patterns becomes increasingly more important.
Servo patterns commonly contain radial transitions of length greater than the width of the writing head. Current methods of writing servo patterns write long radial transitions as an abutment of multiple shorter radial segments. Each shorter segment is written on a separate pass, or disk revolution. Timing of the write operation is derived from a common phase-locked oscillator synchronized to a clock track prerecorded on the disk or, in the case of ‘self-servowriting’ technology, to a fiducial pattern bootstrapped onto the disc by iterative writing by the drive itself.
During currently practiced methods of servowriting, a write head (which commonly is the same head as is used to write user data), is stepped at half servo track pitches, the goal being to provide transitions which radially span two or more adjacent tracks. Typically, a servo track pitch is approximately the width of the recording head. Because quadrature servo position error information is desired on playback, in order to obtain a continuous representation of head position, it is necessary to write track-pitch-long radial transitions both at servo track boundaries, and radially displaced therefrom by half a track width. Presently, these transitions are written in multiple independent passes and are trimmed at a radial midpoint, a strategy imposed by the inability of present writer designs rapidly to toggle write head current in timeframes substantially less than a servo transition interval.
This multi-pass writing approach is a likely source of incoherence, since perfect circumferential transition alignment is typically not achieved owing to mechanical vibration in the head/gimbal assembly and to time jitter in the clock reference source. At high areal densities, transition circumferential incoherence causes a reduction in a signal-to-noise ratio (SNR) associated with servo pattern reading due to edge effects at the transition abutment points. This reduced SNR results in potential read/write head mis-positioning and mis-detection of track address information.
Further, previously written servo pattern transitions often suffer partial erasure effects at the abutment points during servowriting, thereby causing further signal degradation and SNR reduction. These partial erasure effects produce imperfections that increase position error signal modulation leading to reduced tracking accuracy. Additionally, write head drivers embodied in conventional read/write preamplifiers typically have a long write current turn-on and turn-off time compared to the write current transition time. This characteristic prohibits powering the write amplifier on and off quickly enough, especially at high areal densities, to prevent these partial erasure effects. Conventional preamplifiers use a common writer for both data and servo pattern writing functions; power is typically conserved in these designs by switching the writer current source itself, a slow operation owing to significant amount of precision low-speed analog circuitry in the current source. It may be appreciated that the ability to pre-record servo information having sufficient quality presents a major problem area to increasing disk drive performance and increasingly higher servo transition frequencies.
Accordingly, what is needed in the art is a way to reduce incoherence caused by a multiple-pass, independent writing of servo patterns that typically produces an area of circumferential transition misalignment and edge effects between servo pattern half-tracks.